If I remember Heirloom correctly it grants proficiency with a simple or martial weapon?
Then weapon focus requires proficiency with a weapon, that you don't have completely but you do have on that one?

Technically it wouldn't be legal but you GM would be fine allowing it.

Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat. A character can gain a feat at the same level at which he gains the prerequisite.

A character can't use a feat if he loses a prerequisite, but he does not lose the feat itself. If, at a later time, he regains the lost prerequisite, he immediately regains full use of the feat that prerequisite enables.

So Heirloom weapon does give you proficiency in your specific weapon.

If you weapon is a Naginata, you meet the prerequisite, but only when using that weapon. So you can take Weapon Focus(Naginata), but it only applies when wielding your specific weapon. With any other naginata, you do not have proficiency, and thus would lose access to Weapon Focus, and any other feat that has weapon focus as a prerequisite.

Since almost everyone seems to agree on that you can take WF on the specific heirloom weapon "Ascending Dragon" .
If you manage to acquire temporal proficiency with all the rest of nodachis (for instance through the champion spirit of the medium class or the spell Bestow Weapon Proficiency), does the feat apply to all nodachis while you have the proficiency?